The current disclosure pertains to a control system for variable pitch fan engines and turbo-shaft, turbo-propeller engines.
In some fan engines (also known as “propfan” engines), the axis of the fan propeller is parallel to or coaxial with the axis of the gas engine. Typically, in a turbo-shaft, turbo-propeller engine, the axis of one or more propellers will be perpendicular to the axis of the gas engine. In both configurations, the fan or propeller may have a fixed pitch or a variable pitch. If the pitch is variable, the engine may also have a dedicated pitch change mechanism (PCM). The propeller speed (Nr) is proportional to the gas engine power turbine shaft speed (N1) via a pure mechanical gear-train transformation, that is, Nr=Kgb*N1 where Kgb is a constant that represents the gear ratio. Controlling the fan or propeller speed, Nr, is equivalent to controlling the power turbine speed. Generally, the thrust is scheduled to be a function of propeller speed Nr, and also a function of either gas engine high pressure (HP) turbine shaft speed (N2) or engine pressure ratio (EPR). The primary challenge is to coordinate primary control of the propeller speed (Nr), the HP turbine shaft speed (N1), and any PCM pitch angle while maintaining a set of active constraints including but not limited to core pressure (Px), exhaust temperature (T), core speed rate (N2dot), and/or torque (Tq) to stay with defined limits, including but not limited to load change while rejecting external disturbances and/or internal known disturbances including but not limited to variable bleed valves and variable stator vanes.
There remains a need for a systematic control method and a complete control system for a variable pitch fan engine or a turbo-shaft, turbo propeller engine that controls the fan or propeller and the engine coordinately while accommodating the active constraints.